How much space and what are the dimensions you have to work with? Large stock tanks you can find at Farm supply stores work pretty well but are relatively shallow. You could also build one based on some of the Koi pond site information. You might find something at a place that does roto-molding although I'd expect to pay more (I wonder if they have blemished factory seconds?).

My space is unlimited right now. I am putting an addition on my house and will be including the tank and plant growth areas in the addition. I am thinking in the neighborhood of 8 feet in diameter and 4 feet tall for a round tank. If square probably at least a 10 x 4' footprint. Round is cheaper, so I am pretty much expecting to use a round one.

The current set up is an 8 x 3 footprint and its just too small for fish that big. I can't go bigger in the space I have and am putting an addition on anyways... so why not give the fish an upgrade too? I expect to have much more plant growing area as well.

Since I have a few months before I need it, I am trying to buy a used one. Once the addition is built, I will be buying new if I haven't gotten a used one yet.

do you have a way to get a tank that big in to the building after the addition is built or are you going to have to build the addition around the tank?

I've pretty much settled on building plywood tanks with 2x6 or 2x8 etc. as the main structure when I decide to go bigger. Add a pond liner and it's a lot cheaper than buying those huge tanks. If you really want round you could probably build the frame and bend the plywood in to the round shape. Here is an example of a plywood tank... https://backyardtilapia.com/Backyard_Ti ... arming.php

Honestly Rob, no I hadn't thought about doing that. And now I am sitting here wondering how I can justify that I didn't. Putting valves in for draining and filtering wouldn't very realistic, but I can work around that. This would allow me to insulate the sides of the tank easily too.

Wow, this was almost obvious and completely overlooked. I looked hard into doing a fiberglass build with a glass front wall for the existing tank, but finally gave up on it when I found something I could afford. The set up for this will allow viewing from the top easily, and this style enclosure would be ideal.

I wonder if I can get a lighter colored pond liner? Bet I can....Just use a swimming pool liner instead!

My fish are used to having drift wood to swim around, I am not so sure I would want drift wood against this type of liner, I can think of something. Thanks Rob!

a 4ft x 8ft x 4ft plywood tank with pond liner would give you about 957 gallons, and probably cost somewhere in the range of $300 to build if I remember correctly, but I'm not finding where I had saved my calculations to be able to confirm the price I had figured out.

Duraskrim is popular pond liner choice for AP because they are able to get organic ratings using that product whereas other pond liners don't really have the documentation / certifications to be able to get organic ratings (doesn't really matter if you aren't looking to sell your produce, or sell it as organic anyways). They make a white liner to make it easier to see your fish.

As for the driftwood sitting on it... I think the duraskrim is pretty tough stuff... reinforced, so as long as there are no sharp points where it makes contact you'd probably be fine. Otherwise maybe consider putting something else on the bottom where the driftwood would touch.

I used EPDM pond liner for my grow beds which is a lot thicker (and black so would be hard to see fish), but I think there are plenty of cases on here where people used duraskrim in grow beds and its OK having gravel and such sitting on it.

I would probably buy insulation board and insulate the entire tank if I built one... put it on the inside of the plywood to help stabilize temperatures a bit.

with the plywood tank you could pretty easily add Plexiglas viewing panels to view from the side too. Would be the easiest viewing panels added ever because you don't have to have it conform to a round shape =)

just looking at numbers again my $300 estimate is a bit off - probably $300 of lumber/bolts/screws to build a 4x8x4 tank with 1" thick insulation (insulation is about $100 of this so maybe my estimates before I wasn't adding insulation) plus the cost of whatever pond liner you decide to use.

I don't think pool liners are recommended for use in AP if they are exposed to sunlight... I'm not sure if they are OK if it is an indoor system with limited sunlight or not.

I am not sure I trust my fiberglassing skills to do the viewing panel. That was ruled out the last time I upgraded.

Hubby and I have talked about it and we are both liking the idea of a pond liner in a big sturdy box. And the reason I married him is... he says "why 8 x 4 x 4 and not 8 x 8x4?"

He's a keeper! I may go 8 x 6 x 4. Now that I have a better plan for what the fish tank is going to be, its time to meet with the guy that is drawing up the plans for me. (Which is my Dad, kinda handy to have a father that has drawn and built 4 houses. He's too old to build them now, but he can still draw them up!)

Yup I'm torn between 8*8*4 and 8*4*4 too. My next pond will probably be outside so it will probably be 8*8*4 or maybe even a little taller.

For the viewing panel I wouldn't be afraid of cutting out an area to put plexi in the side and just bolt it down to the liner to act as a gasket to seal it. Just use long pieces of plexi on the inside of the plexi window to act as a washer spreading the load so it seals well. I think web4deb had good example of how he did his viewing panel but his we're on round tanks so flat viewing panels should be better to seal

For the viewing panel I wouldn't be afraid of cutting out an area to put plexi in the side and just bolt it down to the liner to act as a gasket to seal it. Just use long pieces of plexi on the inside of the plexi window to act as a washer spreading the load so it seals well. I think web4deb had good example of how he did his viewing panel but his we're on round tanks so flat viewing panels should be better to seal

Ok, you have my attention.... adds to the cost and complexity of the build, but might be worth it. I can totally see how this would work too. You have given me a LOT to think about. Thank you.

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